This invention relates to systems for supplying pressurized fluid to fluid-operated mechanisms. More particularly, the invention relates to fluid supply systems having a variable displacement pump with a compensator that stabilizes the pump output pressure by varying pump displacement in response to a pressure feedback signal from the output.
Variable displacement pump fluid supply systems with a pressure compensator are extensively used to provide driving fluid for hydraulic cylinders and also with other forms of fluid circuit. Basic elements of such a system include a variable displacement pump driven by a motor and drawing fluid from a reservoir. The pump delivers fluid under pressure to any of various mechanisms through a flow control valve which may be closed to stop operation of the driven mechanisms and which may be open to actuate the mechanisms. The compensator responds to a pressure signal, taken from the discharge side of the pump, by increasing pump displacement when discharge pressure drops and by decreasing displacement when discharge pressure rises above a predetermined level and thus tends to stabilize output pressure when changes in the demand for fluid occur.
It is an inherent characteristic of a variable displacement pump that some back leakage of fluid occurs past the pump pistons or other internal mechanisms. In other words, some fluid finds its way back to the supply reservoir through various leakage paths rather than being transmitted to the pump outlet. Steps may be taken to minimize such leakage but as a practical matter some amount of leakage will still occur at least in most forms of pump.
One of the consequences of such leakage is that the pump must have larger pumping capacity than would otherwise be the case. A less obvious but equally serious consequence of leakage is power wastage. Some form of motor, engine or the like is used to drive the pump and the driving motor must consume fuel or otherwise draw upon some energy source. To the extent that the driving motor or the like must deliver power to replace fluid lost through leakage paths, a wastage of power is occurring. Fuel or other energy supplies are uselessly consumed.
Leakage flow within the pump and the consequent power wastage are proportional to the fluid pressure within the pump. Thus the problem of power wastage becomes increasingly severe in higher pressure systems.